1. Field of the Invention
This invention relates to the field of hydrodynamic torque converters for automatic transmissions.
2. Description of the Prior Art
Automatic transmissions of motor vehicles are generally equipped with hydrodynamic torque converters such as are described, for example, in EP-A-0 433 619 or EP-A-0 419 782, or in general terms in Gerick, Bruhn, Danner "Kraftfahrzeugtechnik", Westerman-Verlag, 2nd edition, 1993, pages 349 to 351, the disclosures of which are incorporated herein by reference in their entirety in order to avoid repetition. Hydrodynamic torque converters facilitate soft, smooth starting of a motor vehicle at low engine speeds and gentle shifting of the automatic transmission. Use of these converters leads to a low-noise, low-wear, and infinitely variable transfer of the engine torque to the transmission.
Hydrodynamic torque converters include an impeller connected to the drive shaft, a stator carried on a free-wheel and a turbine connected to the shaft leading into the transmission. The housing of the torque converter, in which the above-mentioned components are arranged, is completely filled by way of a system of passages with a pressure fluid, which in the case of motor vehicles is usually hydraulic transmission fluid. The pressure in the hydrodynamic torque converter is regulated by control valves. The invention will be described below with reference to hydraulic transmission fluid, but is in no way limited to the use of hydraulic transmission fluid as pressure medium.
At low input speeds, the impeller transfers part of the kinetic energy to the hydraulic transmission fluid, which causes the turbine to rotate. By means of the stator, the flow of hydraulic transmission fluid is deflected so as to reinforce the action of the impeller. This shift state of the torque converter is called the "unlock position".
At high speeds, a coupling unit disposed between the impeller connected to the drive and the turbine, the so-called converter lockup clutch; connects the impeller to the turbine nonpositively (so-called "lockup position"), in order to minimize the loss of torque that results as the torque is transferred by means of the rotating stream of oil.
However, due to the nonpositive transfer of the torque in the lockup position, erratic behavior of the drive is transferred directly to the transmission through the lockup clutch, so that the driving behavior of the vehicle is adversely affected.
In order to ensure smooth transfer of the torque in the converter, even in the lockup position, the slip between impeller and turbine can be varied. By means of a control device, comprising a plurality of control valves, the pressure of the hydraulic transmission fluid inside the torque converter is varied, so that the coupling device is opened for a short time. The clutch then slips, i.e., the slip between the impeller and the turbine increases, and sudden, short-time changes in torque due to erratic behavior of the drive can be compensated.
To control the coupling device, the hydrodynamic torque converter is provided with two inlets, controlled by means of a control valve, which are supplied with pressurized hydraulic transmission fluid irrespective of the shift state of the clutch. The first of the two inlets is situated directly on the converter housing, while the second inlet communicates with a space inside the coupling device.
At low speeds of rotation of the turbine, a low pressure is applied to the first inlet of the coupling device, while the second inlet is exposed to a significantly higher pressure. The pressure difference between the inlets leads to separation occurring in the coupling device, which is thereby disengaged. The hydraulic transmission fluid, flowing through the second inlet into the coupling device, flows through the coupling device into the converter housing, and from there, by way of the first inlet, into the control hydraulics circuit (unlock position).
If the point of shift between the unlock and the lockup positions is reached, the first inlet is exposed to a significantly higher pressure than the second inlet. The pressure, now acting in the converter housing, forces the coupling device together, against the pressure acting in the coupling device, so that the impeller is connected nonpositively to the turbine (lockup position).
Throttle bores in the coupling device, or radial channels formed directly in the friction lining of the clutch plates, enable the hydraulic transmission fluid in the converter housing to flow into the space inside the coupling device. The hydraulic transmission fluid can thereby flow onward from the first inlet of the converter by way of the converter housing into the coupling device and thence into the second inlet so that the hydraulic transmission fluid circuit remains closed.
As mentioned above, in order to be able to regulate slip between the impeller and the turbine purposefully, the pressure acting at the first inlet of the converter is varied by means of the control device. This leads to incomplete closure of the coupling device and consequently to partial slippage of the clutch plates.
Since, through the continued movement the hydraulic transmission, fluid is subjected to flexing work and heats up, it must be constantly cooled by means of a cooling system, which is usually located outside the converter. The branching off of hydraulic transmission fluid is described in detail in Automobiltechnische Zeitschrift, 88 (1986), 81-87. The control of the flow of the hydraulic transmission fluid, which in hydrodynamic torque converters serves to drive the turbines and impellers as well as to shift the clutches integrated in the torque converters, takes place, as stated above, through control devices which simultaneously perform several functions.
Thus, for example, DE-A-38 18 102 discloses a control device comprising a lockup clutch control valve having a control plungers a magnetic valve, an oil pump, a control system, and a lubricating device. To operate the lockup clutch, the hydraulic transmission fluid conveyed by the oil pump is supplied by the lockup clutch control valve either to the engaging or to the disengaging chamber of the lockup clutch, according to the shift state of the converter. The position of the control plunger controlling the flow of oil to the lockup clutch is regulated by means of the magnetic valve, which changes a second oil pressure that is independent of the first oil circuit and acts on the end of the control plunger.
While the supply of the lockup clutch with hydraulic transmission fluid takes place through the oil pump, the lockup clutch control valve is controlled by means of a second, independent oil circuit. Apart from additional hydraulic components, such as a further oil pump, a magnetic valve, or further lines, a complicated electronic control system is needed which adapts the various oil circuits to one another.
A further control unit for a lockup clutch of a torque converter is disclosed in DE-C-31 30 871. This control unit consists inter alia of a fluid pressure control unit actuating the lockup clutch, a torque converter control valve, a magnetic valve, a pressure reducing valve unit controlled by a transmission operating mode detector, and an oil pump.
The fluid pressure control unit is responsible for the shift state of the lockup clutch, a valve spindle shifting the oil flow to the engaging or disengaging chamber of the lockup clutch. The torque converter control valve regulates the pressure of the oil that is supplied via the fluid pressure control unit to the lockup clutch, and consequently the slip acting on the lockup clutch.
Independently of this, the position of the plunger of the pressure reducing valve unit is determined by a transmission operating mode detector. The pressure reducing valve unit then controls the position of the valve spindle of the fluid pressure control unit, with a magnetic valve performing the fine adjustment of the valve spindle.
The use of the many different hydraulic shifting components, such as the fluid pressure control unit or the magnetic valve, renders the construction of the control unit very complicated and expensive.
In order to simplify the construction of the control device, control valves have been developed which perform several functions at the same time.
Thus, for example, DE-C-39 28 048 discloses a control device comprising a lockup clutch control valve, a main pressure regulating valve, a magnetic valve, and a "solenoid pressure limit valve". The lockup clutch control valve consists of an electromagnetic shifting component and a valve body with a control plunger slidably arranged therein which opens and closes various inlets and outlets. The control plunger has at its end remote from the electromagnetic shifting component a compression spring, which biases the control plunger towards the electromagnetic shifting component. At the end of the control valve facing the shifting component there is a control pressure chamber, which communicates via a control pressure outlet with the sump of the hydraulic transmission fluid supply and via a control pressure connection with a pump producing the control pressure. The control pressure is kept constant, for example, at 8 bar, by means of the "solenoid pressure limit valve" while the main pressure regulating valve furnishes the pressure acting in the lockup clutch. The magnetic valve generates an effective back pressure of about 1 bar, which acts to prevent the hydraulic transmission fluid flowing out of the converter, in order in this way to load the lockup clutch hydraulically.
By means of the electromagnetic shifting component, either the control pressure outlet can be connected to the sump or the control pressure connection to the control pressure chamber.
If the coupling device is to be opened, the control pressure connection is closed. The compression spring at the end of the control plunger remote from the shifting component pushes the control plunger into its starting position and forces the hydraulic transmission fluid present in the control pressure chamber through the control pressure outlet into the sump of the hydraulic transmission fluid supply. Through the position of the control plunger, the converter clutch back pressure outlet is connected to the supply pressure connection, to which a constant supply pressure is applied, while the converter clutch control pressure connection is connected to the hydraulic transmission fluid cooler. Because of the pressure difference, the coupling device opens.
If the electromagnetic shifting component opens the control pressure connection, it simultaneously closes the control pressure outlet to the sump. The control plunger is thereby displaced against the force of the compression spring, thus connecting the converter clutch control pressure connection to the control pressure connection, while the converter clutch back pressure outlet is connected via a further line to the control pressure outlet leading to the oil sump. The converter clutch is thereby closed.
The electromagnetic shifting component, a so-called pulse-width modulated magnetic valve, is operated at a shifting frequency of, for example, 40 Hz, while the duration of opening per applied control pulse, the so-called pulse width, can be varied. If the pulse width is 100%, the control pressure connection is open for the whole duration of the shifting pulse and the coupling device remains engaged. At a smaller pulse width, i.e., a shorter duration of opening of the control pressure connection, closure of the coupling device is incomplete, i.e., the coupling device slips. In this way, the slip between the turbine and the impeller can be purposefully influenced.
This arrangement has the disadvantage that the different connections have to be supplied with different pressures, which can only be realized through additional hydraulic components such as the solenoid pressure limit valve or the magnetic valve. Through the hydraulic loading, the lockup clutch can only be operated in a pressure range of 1 to 8 bar. A further disadvantage is that the control plunger is only biased by the spring, and at small pulse widths tends to flutter as a result of shifting.